0001313: Image files with greater than 8 bits per channel are always compressed down to 8-bits-per-channel

Description

I think it just needs MagickImageProxy::image to be extended to use ShortPixel if required; the tricky part is knowing the bit depth without reading the image into memory; probably needs to be found by the examiner and passed down that way. Otherwise you need to create a Magick::Image just to get the bit depth (it seems) which gets fiddly quite fast.

@carl: the existing FFmpeg code can load (e.g.) PNGs with no modification; the only question is how to sustain the existing image file / sequence behaviour. With FFmpegDecoder you get a 1-frame-length video.